1. Field of the Invention
The present invention relates to a zoom lens, or a zoom lens barrel, and more specifically to a mechanism for rotating a cam barrel of the zoom lens.
2. Description of the Related Art
In conventional zoom lenses having a plurality of lens groups guided in the direction of the optical axis (i.e., the optical axis direction) of the zoom lens without rotating about the optical axis, a mechanism for moving the plurality of lens groups in the optical axis direction in a predetermined moving manner by rotation of a cam barrel having cam grooves is known in the art. Such a conventional mechanism generally includes a gear formed directly on the outer or inner peripheral surface of the cam barrel, and a motor-driven pinion which is in mesh with the gear of the cam barrel to rotate the cam barrel. Such a conventional mechanism has no substantial adverse effects if it is used for the zoom lens of a conventional camera using light-sensitive film such as 35 mm or APS compact zoom camera.
However, such a mechanism does have substantial adverse effects if it is used for the zoom lens of a digital camera because object images are formed on the sensitive surface of a small CCD (CCD image sensor) which is much smaller than the picture plane of conventional cameras using light sensitive film. Namely, upon assembly, every lens element of a digital camera must be optically centered, correctly spaced, and held firmly with a relatively high precision, e.g., tens times greater than that required in conventional cameras using light-sensitive film. For instance, if the angle of view is constant, the focal length of a photographing lens becomes shorter as the size of the picture plane reduces, which in turn reduces the sizes of all the elements of the photographing lens such as lens elements, lens frames and other elements. Therefore, the influence that a tolerance (e.g., 10 xcexcm) has on a photographing lens system of a digital camera is much larger than the influence that the same tolerance would have on a photographing lens system of a conventional camera using light-sensitive film. Accordingly, manufacturing or assembling error which falls within tolerance of the optical performance in the photographing optical system of a conventional camera using light-sensitive film can be outside the tolerance of optical performance in the photographing optical system of a digital camera. Specifically in the photographing lens system of a digital camera, the influence that an eccentricity or tilt of one or more lens groups has on the optical performance of the photographing optical system is larger than the influence that a deviation of one or more lens groups in the optical axis direction has on the optical performance of the photographing optical system.
From this point of view, in the conventional cam barrel drive mechanism, the cam barrel can be easily deformed minutely since the pinion driven by motor is engaged with the gear formed directly on the cam barrel. In other words, the cam barrel can be easily deformed minutely to cause the rotational center of the cam barrel to deviate from the optical axis of the zoom lens, since the pinion driven by motor is engaged with the gear formed as an integral part of the cam barrel. Once the rotational center of the cam barrel deviates from the optical axis of the zoom lens, the optical centers of the lens groups, whose cam followers are engaged with the cam grooves formed on the cam barrel, also deviate from the optical axis of the zoom lens. Consequently, the optical performance of the zoom lens deteriorates.
The present invention has been devised in view of the above-mentioned problem, and accordingly, an object of the present invention is to provide a mechanism for rotating a cam barrel of a zoom lens, in which the lens groups are moved forward and rearward in the optical axis direction by rotation of the cam barrel having cam grooves, wherein the cam barrel cannot be easily deformed.
To achieve the object mentioned above, according to an aspect of the present invention, a cam barrel rotating mechanism of a zoom lens is provided, including a plurality of lens groups guided in a direction of an optical axis without rotating about the optical axis to change a focal length of the zoom lens; a cam barrel rotatably supported to move the plurality of lens groups in the optical axis direction; a rotational barrel positioned around the cam barrel, the rotational barrel being rotatably driven; a plurality of rotation transmission grooves formed on an outer peripheral surface of the cam barrel, at an equi-angular intervals, extending parallel to the optical axis; and a plurality of inward projections formed on an inner peripheral surface of the rotational barrel to be engaged in the plurality of rotation transmission grooves, respectively.
Preferably, the zoom lens includes a stationary barrel having a female helicoid formed on an inner peripheral surface of the stationary barrel, wherein the cam barrel includes a male helicoid formed on an outer peripheral surface of the cam barrel. The male helicoid of the cam barrel meshes with the female helicoid of the stationary barrel so that the cam barrel moves in the optical axis direction while rotating about the optical axis relative to the stationary barrel.
The plurality of rotation transmission grooves are formed so as to cut across the male helicoid of the cam barrel in a direction parallel to the optical axis.
In an embodiment, the stationary barrel includes a fine male thread, wherein the rotational barrel includes a fine female thread which meshes with the fine male thread so that the rotational barrel moves in the optical axis direction while rotating about the optical axis relative to the stationary barrel. A thread lead of each of the fine male thread and the fine female thread is smaller than a thread lead of each of the male helicoid of the cam barrel and the female helicoid of the stationary barrel.
Preferably, the plurality of inward projections are formed on the rotational barrel at an equi-angular intervals about an axis of the rotational barrel, and the plurality of rotation transmission grooves are formed on the cam barrel at an equi-angular intervals about an axis of the cam barrel.
Preferably, the plurality of inward projections include three inward projections formed on the rotational barrel at 120xc2x0 intervals about the axis of the rotational barrel, and the plurality of rotation transmission grooves include three rotation transmission grooves formed on the cam barrel at 120xc2x0 intervals about the axis of the cam barrel.
Preferably, a depth of each of the plurality of rotation transmission grooves is substantially identical to a depth of thread of the male helicoid. The length of each of the plurality of inward projections in the optical axis direction is determined so that at least part of each of the plurality of inward projections is constantly engaged with an end surface of a portion of thread of the male helicoid in a corresponding rotation transmission groove of the plurality of rotation transmission grooves.
In an embodiment, the zoom lens includes a plurality of lens frames each having a plurality of cam followers which extend radially outwards at an equi-angular intervals about the optical axis; and a plurality of cam grooves which are formed on an inner peripheral surface of the cam barrel, and in which the plurality of cam followers of a corresponding lens frame of the plurality of lens frames are respectively engaged.
The above-described zoom lens can be incorporated in a digital camera.
According to another aspect of the present invention, a zoom lens is provided, including a plurality of lens frames guided in a direction of an optical axis without rotating about the optical axis; a plurality of cam followers formed on each of the plurality of lens frames which extend radially outwards at an equi-angular intervals about the optical axis; a cam barrel having a plurality of cam grooves which are formed on an inner peripheral surface of the cam barrel, the plurality of cam followers being respectively engaged in the plurality of cam grooves, the plurality of lens frames being moved in the optical axis direction without rotating about the optical axis to change a focal length of the zoom lens by rotation of the cam barrel; a rotational barrel positioned around the cam barrel and driven to rotate about the optical axis; a plurality of rotation transmission grooves formed on an outer peripheral surface of the cam barrel at an equi-angular intervals about the optical axis to extend parallel to the optical axis; and a plurality of inward projections formed on an inner peripheral surface of the rotational barrel to be engaged in the plurality of rotation transmission grooves, respectively.
The present disclosure relates to subject matter contained in Japanese Patent Application No. 2000-24041 (filed on Feb. 1, 2000) which is expressly incorporated herein by reference in its entirety.